Over 90% of ethanol biofuel produced in the United States is made from corn starch using Saccharomyces strains to ferment the glucose obtained by hydrolysis of the starch. The United States Environmental Protection Agency has revised the Renewable Fuel Standard (RFS) program as required by the Energy Independence and Security Act of 2007 (EISA). The final rule (RFS2) increases the volume requirements for total renewable fuel to 20.5 billion gallons and for cellulosic biofuel to 3.0 billion gallons by 2015. To meet these mandates, it will be necessary to use cellulosic biomass, an abundant and renewable carbon source, as a feedstock. However, the microbial strains used to ferment the glucose released by hydrolysis of starch are not capable of fermenting the more diverse mixture of sugars released by hydrolysis of lignocellulosic biomass. Saccharomyces strains are generally capable of fermenting the hexose sugars, glucose and galactose; however, they do not naturally ferment the pentose sugars, xylose or arabinose.
To efficiently convert lignocellulo sic biomass to ethanol, it will be necessary to produce a yeast strain capable of utilizing both pentoses and hexoses. The well-studied yeast Scheffersomyces (formerly Pichia) stipitis has the potential to be used more effectively for biomass conversion into ethanol than Saccharomyces strains because it can naturally ferment both pentose and hexose sugars. The strain produces up to 47 g/L ethanol on xylose under conditions of limited aeration and gives ethanol yields up to 0.41 gram/gram on wheat straw hydrolysate. However, S. stipitis has a slower sugar consumption rate than Saccharomyces and requires oxygen for both growth and maximal ethanol production.
Because microaerophilic conditions are difficult to maintain uniformly in large-scale industrial fuel ethanol operations, enhancing the capability of this yeast to produce ethanol anaerobically could increase its value in industrial processes. As such, there is a need develop Scheffersomyces stipitis yeast strains that produce ethanol that can ferment hexose sugars and pentose sugars in large-scale industrial fuel ethanol operating conditions.